A Braided-Reinforced Composite Hollow Fiber Membrane

ABSTRACT

A braid-reinforced composite hollow fiber membrane is disclosed. The braid-reinforced composite hollow fiber membrane comprising a reinforcing material of a tubular braid and a polymer resinous thin film coated on the surface of the tubular braid according to the present invention is characterized in that: the tubular braid comprises multifilaments made of monofilaments having a crimp rate of 2 to 40%, and the peeling strength of the tubular braid and a polymer resinous thin film coated on the surface thereof is 1 to 10 MPa. In the composite hollow fiber membrane, the crimp rate of the monofilaments constituting the tubular braid of the reinforcing material is 2 to 40%, thus the surface area of the tubular braid contacted with the polymer resinous thin film is increased. Thus, the peeling strength of the tubular braid and the polymer resinous thin film coated on the surface thereof is excellent.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a composite hollow fiber membranehaving an excellent peeling strength, filtration reliability, and waterpermeability.

Recently, polymer separation membranes are being utilized in morevarious fields as well as existing application fields with theimprovement of their techniques. Particularly, with the importance ofenvironment, demands for them are being increased in the fields of watertreatment. In all application fields of separation membranes, amechanical strength, such as peeling strength, always stands out as animportant factor as well as selectiveness and water permeability.Particularly, in water treatment fields, an excellent mechanicalstrength is necessarily required, simultaneously with a highpermeability, from the viewpoint of the reliability of a separationmembrane system.

2. Background Art

A hollow fiber-shaped membrane has a high permeability per installationarea and is suitable for water treatment, whereas the mechanicalstrength thereof has been a problem to be solved due to thecharacteristics of a porous membrane structure. Thus, a hollow fibermembrane reinforced with a fabric or tubular braid having an excellentmechanical strength as a support of the separation membrane. Such ageneral idea of a composite membrane is a well known fact.

Techniques thereof are disclosed in U.S. Pat. No. 4,061,821, U.S. Pat.No. 3,644,139, U.S. Pat. No. 5,472,607, No. 6,354,444 and the like.Among them, a general idea of a composite hollow fiber membrane using atubular braid was disclosed for the first time in U.S. Pat. No.4,061,821 to Hayano et al. In this technique, however, the tubular braidis not used as a support for coating, but it is completely embedded inthe membrane in order to compensate for a reduction of waterpermeability due to the shrinkage occurred when an acryl hollow fibertype membrane is solely used at a temperature higher than 80° C. Such acomposite membrane has a larger thickness than the thin film coated on asupport, and the embedded braid increases the resistance of fluid flowfor thereby significantly reducing the water permeability. Unlike theprior art, in U.S. Pat. No. 5,472,607, a reinforcing material is notembedded in the membrane, but is coated on its surface with a thin filmby coating method of the existing flat composite membrane. Inmanufacturing a composite hollow fiber membrane having a thin film layercoated on the surface of a reinforcing material or supporting materialof a tubular braid, thermodynamic stability differs according to thecomposition of a dope to be used for coating. This determines thestructure of the coated thin film layer.

That is to say, in case of a thermodynamically stable dope, it has afinger-like structure. On the contrary, a dope with a low thermodynamicstability has a sponge structure with no defect region. For instance, incase of a dope used a solvent having a strong solvent power such asN-methyl-2-pyrrolidone (NMP) among organic solvents, it can easily forma finger-type structure because it has a high thermodynamic stability.

Additionally, the water permeability and mechanical strength of theoverall composite hollow fiber membrane depends upon the structure andproperties of the thin film layer. This is because the thin film layerhas small pores and a low mechanical strength than a tubular braidreinforcing material having relatively much larger pores and a higherstrength. In other words, the filtrate having passed through the thinfilm layer passes through a braid supporting layer with relatively largepores without a large resistance. While, since the thin film layer has alarge flow resistance, the water permeability of the overall membrane isdetermined according to a microporous structure and porosity.

In view of strength, the tensile strength, pressure resistance and thelike are complemented by the braid reinforcing material having a farsuperior mechanical strength. However, if the strength of the thin filmis reduced, the thin film is separated or damaged.

In U.S. Pat. No. 4,061,821 and U.S. Pat. No. 5,472,607, the significanceof the coated thin film layer structure was overlooked in relative tothe present invention. Particularly, the structure of the thin filmlayer in the two prior arts has a porous region larger than 5 μm in aninner layer of a skin, that is, the inner layer has some micro poreshaving a pore diameter larger than 5 μm.

FIG. 2 is an exploded sectional view of a composite hollow fibermembrane disclosed in U.S. Pat. No. 4,061,821; and FIG. 3 is an explodedsectional view of a composite hollow fiber membrane disclosed in U.S.Pat. No. 5,472,607. These membranes are in a finger-like structure asshown in FIGS. 2 and 3 and have a defect region D functioning as adefect in the thin film layer.

As seen from the well-known fact, they can acts as a defect inexpressing the mechanical properties of the thin film. Particularly,when the skin of a dense layer is damaged, a material capable of beingsecondarily cut off by the inner layer is permeated. This reduces thefiltration reliability of the membrane relatively.

The composite hollow fiber membrane is suitable, particularly forfiltration modules in the fields of water treatment due to its superiormechanical strength. In such a filtration module, there is a possibilityof damaging the surface of the membrane by the friction and physicalimpact generated between membranes due to aeration. Particularly,filtration by the inner layer is required so as to ensure highfiltration reliability.

Meanwhile, U.S. Pat. No. 6,354,444 proposes a composite hollow fibermembrane coated with a polymer resinous thin film on a braid madecomposite hollow fiber membrane, the braid is made of monofilaments of0.5 or higher denier without crimp, so the surface area of the braidcontacted with the polymer resinous thin film is small, which leads to alow peeling strength between the braid and the polymer resin coated onits surface.

DETAILED DESCRIPTION OF THE INVENTION Problem to be Solved by theInvention

It is an object of the present invention to provide a composite hollowfiber membrane having an excellent peeling strength, filtrationreliability, and water permeability by coating a polymer resinous thinfilm on the support of a braid.

The present invention provides a composite hollow fiber membrane whichincreases the surface area of a tubular braid contacted with a polymerresinous thin film and has excellent peeling strength of a tubular braidand a polymer resinous thin film coated on the surface thereof by givinga crimp rate of a predetermined range to the monofilaments constitutingthe tubular braid and making the monofilaments superfine.

Technical Solution

The braid-reinforced composite hollow fiber membrane comprising areinforcing material of a tubular braid and a polymer resinous thin filmcoated on the surface of the tubular braid according to the presentinvention is characterized in that: the tubular braid comprisesmultifilaments made of monofilaments having a crimp rate of 2 to 40%,and the peeling strength of the tubular braid and a polymer resinousthin film coated on the surface thereof is 1 to 10 MPa.

The present invention will now be described in detail with reference tothe accompanying drawings.

The composite hollow fiber membrane of the present invention has astructure in which a polymer resinous thin film (A) is coated on thesurface of the reinforcing material of a tubular braid (B). FIG. 1 is aschematic cross sectional view of a composite hollow fiber membraneaccording to the present invention.

In the present invention, the tubular braid comprises multifilamentsconsisting of monofilaments having a crimp rate of 2 to 40% and, thusthe peeling strength of the tubular braid and the polymer resinous thinfilm coated on the surface thereof is very excellent, that is, 1 to 10MPa.

If the crimp rate is less than 2%, the surface area of the tubular braid(B) contacted with the polymer resinous thin film (A) is reduced, whichdecrease the peeling strength to less than 1 MPa. If the crimp rate isgreater than 300%, the process stability in the production of a tubularbraid is decreased.

Preferably, the tubular braid (B) comprises multifilaments consisting ofmonofilaments having a fineness of 0.01 to 0.4 denier.

If the fineness of the monofilaments is above 0.4 denier, the surfacearea of the tubular braid (B) contacted with the polymer resinous thinfilm is reduced, which leads to a low peeling strength of less than 1MPa between the tubular braid (B) and the polymer resinous thin film (A)coated on its surface.

In addition, if the fineness of the monofilaments is less than 0.01denier, the initial wetting property and peeling strength of the tubularbraid (B) and the polymer resinous thin film (A) are improved, but themanufacturing process becomes complicated and the manufacturing costsare raised.

Preferably, the multifilaments constituting the tubular braid (B)consist of 150 to 7,000 monofilaments and have a total fineness of 30 to140 deniers.

Preferably, the tubular braid (B) is knit by using 16 to 60 gray yarnsfor knitting prepared by combining 4 to 10 multifilaments.

The polymer resinous thin film comprises a skin layer of a densestructure and an inner layer of a sponge structure. The skin layer isformed with micro pores having a diameter in the range from 0.01 to 1μm. The inner layer is formed with micro pores having a diameter lessthan 10 μm, preferably, 5 μm.

The present invention is characterized in that it has no defect regionlarger than 10 μm in the inner layer of the polymer resinous thin film,that is, there exist no micro pores having a diameter larger than 10 μm.

In a case that any defect region larger than 10 μm exists in the innerlayer, the filtration reliability can be reduced greatly. Preferably,the diameters of micro pores formed in the inner layer of the spongestructure are continuously and gradually increased with the approach tothe central direction of the composite hollow fiber membrane.

To improve both mechanical strength and water permeability, it ispreferable that the thickness of the polymer resinous thin film is lessthan 0.2 mm and the penetrating length of the polymer resinous thin filminto the reinforcing material is less than 30% of the reinforcingmaterial thickness.

The polymer resinous thin film is made from a spinning dope consistingof polymer resin, organic solvent, polyvinylpyrrolidone and hydrophiliccompound.

The composite hollow fiber membrane of the present invention can be madeby passing a tubular braid (reinforcing material) through the centerportion of a double tubular nozzle and simultaneously feeding a spinningdope for the polymer resinous thin film on the surface of the braidthrough the nozzle, coating the spinning dope on the braid, extrudingthem in the air of outside the nozzle, coagulating them in a externalcoagulating liquid to form the composite hollow fiber membranestructure, and washing and drying it.

At this time, the spinning dope for the polymer resinous thin film isobtained by dissolving polymer resin, polyvinylpyrrolidone andhydrophilic compound in an organic solvent. More preferably, thespinning dope is made of polymer resin of 10 to 50% by weight,polyvinylpyrrolidone and a hydrophilic compound of 9 to 30% by weightand an organic solvent of 20 to 89% by weight. However, in the presentinvention, the composition ratio of the spinning dope is notspecifically limited.

The polymer resin is polysulfone resin, polyethersulfone resin,sulfonated polysulfone resin, polyvinylidenefluoride (PVDF) resin,polyacrylonitrile (PAN) resin, polyimide resin, polyamideimide resin,polyetherimide resin and so on. The organic solvent is dimethylacetamide, dimethyl formamide or a mixture thereof.

The hydrophilic compound is water or glycol compound, and morepreferably, polyethylene glycol having a molecular weight less than2,000. Since the water or glycol compound, which is hydrophilic, reducesthe stability of the spinning dope, it is more likely to form a spongestructure relatively.

That is, as the stability of the spinning dope becomes higher, it ismore likely to form a finger-like structure because a defect region(micro pores having a diameter larger than 10 μm) is formed in themembrane. The present invention reduces the stability of the spinningdope by adding water or glycol compound, an additive, simultaneously toincrease the water permeability by making the membrane hydrophilic.

Meanwhile, in the process of producing the composite hollow fibermembrane, in order to uniformly coat a polymer resinous thin film on thesurface of the reinforcing material of the tubular braid at apredetermined thickness, the speed with which the tubular braid isadvanced and the quantity of the spinning dope introduced into thenozzle must be balanced with each other. The relation between the feedrate (Q) of a spinning dope and the speed (u) of a tubular braid isexpressed by the formula

Q=πρνD_(o)T

[wherein Q denotes the feed rate of dope per hour, ρ denotes the densityof dope, ν denotes the advancing speed of the braid, D_(o) denotes theouter diameter of the braid and T denotes the thickness of the dope tobe coated.]

As seen from the above formula, in case that the advancing speed of thebraid is high, a thin coating layer is formed. In case that theadvancing speed of the braid is extremely higher in relative to the feedrate of the spinning dope, a non-uniform membrane with no coating layeron some parts is produced. Otherwise, a non-uniform membrane with apartially thick coating layer is produced. That is, it can be known thatthere exists an optimum speed ratio for stably producing a membrane witha uniform thickness.

In addition, the polymer resinous thin film of the composite hollowfiber membrane according to the present invention comprises a dense skinlayer and an inner layer of a sponge structure whose pore diameterbecomes gradually larger with the approach to the center of thecomposite hollow fiber membrane

Due to this, the composite hollow fiber membrane of the presentinvention has an excellent peeling strength, initial wetting property,filtration reliability and water permeability.

In the present invention, the physical properties of the compositehollow fiber membrane are evaluated by the following method.

Crimp Rate

First, a composite hollow fiber membrane used as a sample is woundaround a denier creel 10 times to prepare a bundle of yarns. At thistime, the winding tensile force is equal to (nominal fineness× 1/10 g).

An initial load (nominal fineness× 1/20 g) and a static load (nominalfineness×2 g) are suspended on the produced bundle of yarns and put intowater (20° C.±2° C.) and left therein for two minutes, and then thelength L0 of the sample is measured.

The sample is taken out and only the static load is removed, and thenput into the water again and left therein for three minutes, and withthe same being put in the water, the length L1 of the sample ismeasured.

The measured lengths L0 and L1 of the sample were substituted into thefollowing formula, to thus calculate (Cr, %):

${{Crimp}\mspace{14mu} {{Rate}\left( {{Cr},\%} \right)}} = {\frac{{L\; 0} - {L\; 1}}{L\; 0} \times 100\%}$

Peeling Strength

The load at the instant when a coated polymer resinous thin film ispeeled from a tubular braid by using a tensile tester was measured anddivided into the area m² to which shear strength is applied to thuscalculate the peeling strength.

Specific measurement conditions are as follows.

-   -   measuring instrument: Instron 4303    -   load cell: 1 KN    -   crosshead speed: 25 mm/min    -   sample: The sample was produced by bonding and securing one        strand of a composite hollow fiber membrane to a polypropylene        tube having a 6 mm diameter by using polyurethane resin so that        the length of the bonding portion should be 10 mm.

${{Peeling}\mspace{14mu} {{Strength}({Pa})}} = \frac{{load}\mspace{14mu} {of}\mspace{14mu} {yield}\mspace{14mu} {{point}({kg})}}{{application}\mspace{14mu} {area}\mspace{14mu} {of}\mspace{14mu} {shear}\mspace{14mu} {{strength}\left( m^{2} \right)}}$

The peeling strength is defined as the shear strength per unit areaapplied to a coated polymer resinous thin film when the sample isextended.

The application area (m²) of the shear strength is calculated by theformula: π X outer diameter of composite hollow fiber membrane X lengthof bonding portion of composite hollow fiber membrane.

Water Permeability

The water permeability was measured by preparing a mini-module having aneffective length of 10 cm in the composite hollow fiber membrane andpassing pure water through the module for a predetermined time by out-inflow method under a suction pressure of 1 kg at a temperature of 25° C.

${{Peeling}\mspace{14mu} {{Strength}({Pa})}} = \frac{{load}\mspace{14mu} {of}\mspace{14mu} {yield}\mspace{14mu} {{point}({kg})}}{{application}\mspace{14mu} {area}\mspace{14mu} {of}\mspace{14mu} {shear}\mspace{14mu} {{strength}\left( m^{2} \right)}}$

Shape of Micro Pores

The fracture surface of the polymer resinous thin film layer coated onthe surface of the support (reinforcing material) was observed with ascanning electron microscope.

The composite hollow fiber membrane of the present invention produced bythe above method has an excellent initial wetting property of 80 to 120%and an excellent peeling strength of 1 to 10 MPa between the tubularbraid (B) and the polymer resinous film (A) coated on its surface.

EFFECT OF THE INVENTION

The peeling strength of the tubular braid and the polymer resinous thinfilm coated on the surface thereof is excellent, and at the same time,the initial wetting property of the composite hollow fiber membrane isexcellent.

The composite hollow fiber membrane of the present invention isreinforced with a support of a braid and has no defect region greaterthan 10 μm in the inner layer (sponge structure) of the polymer resinousthin film (no micro pores greater than 10 μm formed in the inner layer.Therefore, the water permeability, mechanical strength and filtrationreliability thereof are excellent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross sectional view of a composite hollow fibermembrane according to the present invention; and

FIGS. 2 and 3 are enlarged cross sectional views of conventional hollowfiber membranes.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is now understood more concretely by comparisonbetween examples of the present invention and comparative examples.However, the present invention is not limited to such examples.

Example 1

A spinning dope is prepared from components: 17% by weight ofpolysulfone, 9% by weight of polyvinylpyrrolidone, and 10% by weight ofpolyethyleneglycol added to 64% by weight of dimethylformamide (organicsolvent), to produce a transparent spinning dope by mixing anddissolving the components. The spinning dope is fed into a doubletubular nozzle having a 2.38 mmφ diameter and simultaneously a tubularbraid, which is knit to have an outer diameter of 2 mm by using 12 grayyarns for knitting prepared by combining 6 multifilaments having a crimprate of 10% and a fineness of 80 deniers comprising 200 strands ofmonofilaments having a fineness of 0.4 denier, is passed through thecenter portion of the nozzle, to thus coat the spinning dope on thesurface of the tubular braid and then extrude it in the air. At thistime, the ratio (k) of the advancing speed of the braid to the feed rateof the spinning dope is 750 g/m², and the coating thickness of thespinning dope is 0.2 mm. After passing through the tubular braid coatedwith the spinning dope into a 10 cm air gap, it is coagulated in anexternal coagulating bath with a temperature of 35° C. Subsequently, thecomposite hollow fiber membrane is prepared by washing in a washing tankand winding. The result of evaluation for the structure and physicalproperties of the produced composite hollow fiber membrane is shown inTable 1.

Example 2

A composite hollow fiber membrane is produced in the same process andcondition as Example 1, except that a tubular braid is used, which isknit to have an outer diameter of 2 mm by using 12 gray yarns for 110knitting prepared by combining 6 multifilaments having a crimp rate of20% and a fineness of 65 deniers comprising 650 strands of monofilamentshaving a fineness of 0.1 denier. The result of evaluation for thestructure and physical properties of the produced composite hollow fibermembrane is shown in Table 1.

Comparative Example 1

A composite hollow fiber membrane is produced in the same process andcondition as Example 1, except that a tubular braid is used, which isknit to have an outer diameter of 2 mm by using 16 gray yarns forknitting prepared by 3 multifilaments having a crimp rate of 0% and afineness of 150 deniers comprising 300 strands of monofilaments having afineness of 0.5 denier. The result of evaluation for the structure andphysical properties of the produced composite hollow fiber membrane isshown in Table 1.

TABLE 1 Result of Physical Properties of Composite Hollow Fiber MembraneComparative Classification Example 1 Example 2 Example 2 PeelingStrength (MPa) 1.65 3.29 0.85 Crimp rate (%) of 10 20 0 monofilaments intubular braid

INDUSTRIAL APPLICABILITY

The peeling strength of the tubular braid and the polymer resinous thinfilm coated on the surface thereof is excellent, and at the same time,the filtration reliability and water permeability of the compositehollow fiber membrane are excellent.

The composite hollow fiber membrane of the present invention isreinforced with a support of a braid and has no defect region greaterthan 10 μm in the inner layer (sponge structure) of the polymer resinousthin film (no micro pores greater than 10 μm formed in the inner layer.Therefore, the water permeability, mechanical strength and filtrationreliability thereof are excellent. As the result, the composite hollowfiber membrane of the present invention is particularly suitable forfiltration modules in the fields of water treatment of a large size.

1. A braid-reinforced composite hollow fiber membrane comprising areinforcing material of a tubular braid and a polymer resinous thin filmcoated on the surface of the tubular braid characterized in that: thetubular braid comprises multifilaments made of monofilaments having acrimp rate of 2 to 40%, and the peeling strength of the tubular braidand a polymer resinous thin film coated on the surface thereof is 1 to10 MPa.
 2. The braid-reinforced composite hollow fiber membrane of claim1, wherein the fineness of the multifilaments is 0.01 to 0.4 denier. 3.The braid-reinforced composite hollow fiber membrane of claim 1, whereinthe multifilaments constituting the tubular braid consist of 150 to7,000 monofilaments.
 4. The braid-reinforced composite hollow fibermembrane of claim 1, wherein the fineness of the multifilaments is 30 to140 deniers.
 5. The braid-reinforced composite hollow fiber membrane ofclaim 1, wherein the tubular braid is knit by using 16 to 60 gray yarnsfor knitting prepared by combining 4 to 10 multifilaments.
 6. Thebraid-reinforced composite hollow fiber membrane of claim 1, wherein thethickness of the polymer resinous thin film is less than 0.2 mm.
 7. Thebraid-reinforced composite hollow fiber membrane of claim 1, wherein thepenetrating length of the polymer resinous thin film into thereinforcing material is less than 30% of the reinforcing materialthickness.
 8. The braid-reinforced composite hollow fiber membrane ofclaim 1, wherein the polymer resinous thin film is made from a spinningdope consisting of polymer resin, organic solvent, polyvinylpyrrolidoneand hydrophilic compound.
 9. The braid-reinforced composite hollow fibermembrane of claim 1, characterized in that the polymer resin ispolysulfone resin, polyethersulfone resin, sulfonated polysulfone resin,polyvinylidenefluoride (PVDF) resin, polyacrylonitrile (PAN) resin,polyimide resin, polyamideimide resin or polyetherimide resin.
 10. Thebraid-reinforced composite hollow fiber membrane of claim 7, wherein thehydrophilic compound is water or glycol compound.
 11. Thebraid-reinforced composite hollow fiber membrane of claim 10, whereinthe glycol compound is polyethylene glycol having a molecular weightless than 2,000.
 12. The braid-reinforced composite hollow fibermembrane of claim 7, wherein the organic solvent includes dimethylacetamide, dimethyl formamide or a mixture thereof.
 13. Thebraid-reinforced composite hollow fiber membrane of claim 7,characterized in that the polymer resin is polysulfone resin,polyethersulfone resin, sulfonated polysulfone resin,polyvinylidenefluoride (PVDF) resin, polyacrylonitrile (PAN) resin,polyimide resin, polyamideimide resin or polyetherimide resin.